When Mochizuki Hidemitsu, Lord of Azuma and head of one of the great 13 Houses, dies under suspicious circumstances and the lord governing the neighboring Asaka, Ashiya Tatsuyoshi, tries to force Hidemitsu's daughter, Sayo-hime, to marry him, she asks the shinobi of Kasumidanishuu to investigate the death.
While they manage to uncover the truth about the death, this ultimately leads to the annihilation of Kasumidanishuu on the hands of the Meigokutou, a group of Shinobi allied to Ashiya.
The survivors rally around Yashiro Jinma, who becomes the leader of the group that Sayo-hime dubs the Yamabukigumi.

Asaka has one of the largest army in the Dragon Realm as well the support of the Meigokutou and apparently the tacit approval of the shogunate.
On the other hand the retainers of the Mochizuki house are split on the question whether to fight or to negotiate, is much less militarily focused and the Yamabukigumi is small and disorganized.
But they have even stronger cards...